Armored Scales

Armored scales are rarely a problem on avocados in California. If you see high numbers of armored scales on avocados, contact your local agricultural commissioner, as it is quite possibly an exotic species introduced into the state.

Armored scales (family Diaspididae) have a flattened, slightly convex cover that at maturity is about 0.06 inch (3 mm) in diameter. This platelike cover usually can be removed to reveal the actual scale body underneath. Armored scale covers typically have a different colored, slight protuberance (exuviae or "nipple") and concentric rings, which form as each nymphal stage enlarges its cover. Females develop roundish covers. For species with males, their covers are elongate in late instars.

Latania and greedy scale can reliably be distinguished only by an expert. Their covers are gray, tan, or white. Dictyospermum scale has a yellowish brown cover that is somewhat darker than the similar-looking, orange to reddish California red scale cover. California red scale and latania scale occur throughout the plant, with relatively even distribution among fruit, leaves, and wood. Dictyospermum scale infests mostly fruit and leaves. Greedy scale is usually limited to twigs and branches.

Latania scale and greedy scale females lay eggs beneath their cover, from which crawlers hatch. California red scale and dictyospermum scale give live birth to young crawlers. Greedy scale and latania scale reproduce without males, at least in California. Both California red scale and dictyospermum scale produce males, which as immatures develop under elongate covers.

Scales in avocado are usually under good biological control. Latania scale occasionally damages avocado. Large numbers of latania scale on bark can kill twigs, especially on young trees. Unlike many plant-sucking insects, armored scales do not secrete any noticeable liquid. Economic damage is from scale covers on the fruit skin, which appear as tiny dimples or light-colored spots. Feeding may also cause small discolored spots in the skin. Internal fruit quality is not impaired, but infested or spotted fruit may be culled. California red scale is a rare problem, and only on avocado near citrus. Dictyospermum scale and greedy scale occur in avocado only at very low numbers.

Biological control is the primary scale control method in avocados. Conserve natural enemies by minimizing dust and avoiding application of broad-spectrum, persistent insecticides that promote pest flare-ups because they kill natural enemies. If certain areas of a grove have high armored scale numbers, determine whether encrusted fruit can be selectively harvested and sent to a packing house that uses brushes or pressure-washing equipment that can remove scale covers from fruit. In the infrequent event that direct control may be justified, oil spray has little long-term adverse impact on natural enemies. Time any scale treatments to occur soon after most scale crawlers have emerged.

Predatory insects and parasitic wasps control most scales. Armored scale parasites include species of tiny Aphytis and Aspidiotiphagus (family Aphelinidae), and Plagiomerus, Comperiella and Signiphora (family Encyrtidae). Most scale predators feed on both armored and soft scales and often on other pests. Predators include brown and green lacewings, pirate bugs, predaceous mites such as Cheletomimus berlesei and Hemisarcoptes malus, and Karnyothrips species.

Predaceous Coccinellidae include the spotless lady beetle (Cycloneda sanguinea), steelblue lady beetle (Halmus chalybeus), and twicestabbed lady beetle (Chilocorus orbus =C. stigma). As adults, these lady beetles are about 0.16 to 0.2 inch (4–5 mm) long.

• Spotless lady beetle has a black and white head and thorax and orangish wing covers without markings.
• Steelblue lady beetle is metallic bluish.
• Twicestabbed lady beetle is shiny black with two large orangish spots on its wing covers. Its larvae are black to brownish with a yellowish transverse band and are covered with branched spines.

Use biological and cultural controls and some oil sprays on an organically certified crop.

In the rare situation where treatment is warranted, spray oil after the end of maximum crawler emergence. To time an application, monitor scale crawlers by trapping them with transparent tape that is sticky on both sides. Wrap tape traps tightly to encircle each of several twigs near female scales. Replace traps weekly when crawlers are expected. Preserve traps sandwiched between clear plastic and light blue paper, and label papers with the trap date and location. Visually compare crawler abundance in traps among monitoring dates. Treat when it is obvious that more crawlers per trap were caught during previous weeks and catches have definitely declined. If persistent populations of California red scale are present, consider releasing a small number (perhaps 10,000) of Aphytis melinus near the scale infested trees after purchasing them from an insectary.